target: Fix residual overflow handling in target_complete_cmd_with_length

This patch fixes residual overflow handling to correctly set the
residual_count using SPDTL, instead of SCSI Allocation Length.

Allocation Length is the maximum value of the SPDTL and not substitute
for it, hence it shouldn’t be used to calculate ResidualCount except for
cases where SPDTL > Allocation Length and Data is truncated (in that
case both Alloc Len and SPDTL are same). (SPC 5r01 Section 4.2.5.6).

Thanks to Ajay Nair in assisting with this patch.

Signed-off-by: Sumit Rai <sumitrai96@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>